Rotor assemblies of turbomachinery, such as gas turbine engines, are comprised of rows of rotor disks, each having attached about its periphery a distributed array of blades. The typical method of attaching the blades to the disks is to provide side-entry, dovetail slots into which the dovetail root sections of the blades are slidingly received and captured. During engine operation, relative sliding motion between the blade dovetails and the disk dovetail slots occurs due to centrifugal loading which causes the disk outer diameter to grow larger allowing the blades to slide radially outward. Associated with this blade loading and sliding motion is a resisting frictional shearing force on the interfacial surfaces of the blade and slot dovetails. In addition, Hertzian contact stresses are developed at the interface of the contacting surfaces due to the normal crush and frictional shear loads. The magnitudes of these stresses depend on the dovetail geometries of the blade and slot, interface coefficient of friction, and blade loading. The surface stresses associated with normal crush loads are compressive, while those associated with frictional shear can be either tensile or compressive depending on the direction of shear loading.
For high thrust engines utilizing highly loaded blades, the tensile surface stresses associated with frictional shear loads have been found through analysis and actual testing to be quite high. These high stresses, when applied cyclically as during normal engine operation, can exceed the fatigue capabilities of the fan blade and/or disk, resulting in the creation of cracks.
To relieve surface stresses due to frictional shear, attention has been directed to minimizing the friction over the interface surfaces of the blade and slot by the application of a dry film lubricant, such as molydisulfide. This approach is also beneficial in reducing fretting or galling of the blade and slot dovetail surfaces as they rub against each other. Fretting has been found to have a particularly negative impact on low cycle fatigue (LCF) capability due to the potential for initiating premature cracking. While lubrication of the interface surfaces does reduce coefficient of friction, this beneficial effect is not lasting due to degradation under the harsh operating environment of a gas turbine engine. Periodic relubrication is thus required to reestablish the desired low interface surface coefficient of friction. Frequent maintenance of this type is inconvenient and costly.
Another approach to relieving frictional stresses along the blade-disk interface has been to reduce normal crush stress by changes in slot geometry This can be accomplished by increasing the contact area of the interface. However, this requires increasing the size of the blade and slot dovetails, resulting in heavier fan blades and rotor disk. This is contrary to the current design emphasis of reducing engine weight. Another proposed geometry change is to reduce the pressure face angle of the blade and slot dovetail contacting surfaces. However, this results in higher normal loads at their interface and has the further disadvantage of increasing the stress in the dovetail slot fillet radius due to increased bending stress on the disk material between the slots.
To improve gas turbine engine performance in commercial and military aircraft, the design emphasis is directed toward increasing operating speeds, temperatures and pressures while also reducing engine weight. This results in increased centrifugal loading of the fan blades and thus excerbation of the surface stress problem in the dovetail slot with attendant reduction in low cycle fatigue life.